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## Algebra 1

### Unit 11: Lesson 3

Simplifying square roots# Simplifying square roots

CCSS.Math:

Roots are nice, but we prefer dealing with regular numbers as much as possible. So, for example, instead of √4 we prefer dealing with 2. What about roots that aren't equal to an integer, like √20? Still, we can write 20 as 4⋅5 and then use known properties to write √(4⋅5) as √4⋅√5, which is 2√5. We *simplified* √20. Created by Sal Khan.

## Want to join the conversation?

- At0:28, you added all the values and observed that if the sum was divisible by 3, so was the value. What video can I find this principle?(168 votes)
- The link is broken, here is the new one:

https://www.khanacademy.org/math/pre-algebra/factors-multiples/divisibility_tests/v/divisibility-tests-for-2-3-4-5-6-9-10(104 votes)

- At0:09, Sal said that 117 is not a perfect square. What does that mean?(34 votes)
- A perfect square is a square root is not a decimal. You can not take the square root of 117 and have it not be a decimal. But if you were to take the square root of 9, it would be 3 because 3x3=9. Hope this helped!(46 votes)

- Wouldn't the answer to a square root really be positive and negative? For instance, if we wanted the square root of 9, it would be 3 and -3? because 3x3=9 and -3x-3=9?(34 votes)
- Yes, whenever you take square roots, you get two values (one positive and the other negative). But when you take the "principle" square root , you take only the positive value.(38 votes)

- I'm having a LOT of trouble siplifying square roots and I can't understand why it's not making any sense to me...

The Square Roots Practice I can finish in about 10 seconds but I'm really hitting a wall with the Simplification side of Square Roots. Please help me!(33 votes) - Around2:24, Sal explains that 5*3 and the square root of thirteen is 15 times the square root of thirteen. Why would you multiply the numbers 5 and 3?(27 votes)
- He is trying to simplify it. 5•3•√13 is more complex than 15•√13. The former has 3 steps involved (multiply 5 and 3, find square root of 13, multiply 15 by square root of 13), while the latter only has 2 steps involved (find square root of 13 and multiply by 15).(30 votes)

- Which video (and where) explains why you can add up the digits of a number to see if it's divisible by 3 like at0:25-0:36?(19 votes)
- go to pre- algabra and in the factors and multiples section you will find divisablity tests at the top of the list and it explains the rule for 3 in the first video(26 votes)

- what is the concept of simplifying square roots? I don't understand square roots(16 votes)
- Roots are the inverse operation to powers. So if you take the square root of 6 and then you square it, then you would be left with 6 because the square and the square root cancel out.

Now if you have the square root of 2 plus the square root of 2, you would have 2√2. Notice that it isn't √4. It is actually 2√2 (which is the same as √8).

So the concept of simplifying square roots is like the concept of simplifying other things like exponents, parentheses, etc.(5 votes)

- Okay so how would you do fractions? I'm very confused and my math teacher sped through it so I didn't understand. How would you simplify the sqare root of 35 over 9 (just and example)?(11 votes)
- The thing about a square root of a fraction is that:

sqrt(35/9) = sqrt(35)/sqrt(9)

in other words, the square root of the entire fraction is the same as the square root of the numerator divided by the square root of the denominator. With that in mind, we can simplify the fraction:

sqrt(35)/3

As you can see, I left the numerator under the square root, because I can't simplify it, but the square root of 9 is three so I could replace the sqrt(9) in the denominator by 3.

The same rule applies to exponents: e.g. (2/3)^2=(2^2)/(3^2)(12 votes)

- i still don't understand the concept(9 votes)
- I found a website that breaks this concept down as if they were teaching it to kindergarten students XD

https://www.mathsisfun.com/numbers/simplify-square-roots.html

Helped me finally understand this!(9 votes)

- I don't understand. Why do you keep 3 times the square rootof 26 the same, and not make it 3 times the square root of 2 times 13?(8 votes)
- because there is no point in writing 2 times 13 if you can't write the square root of either. The whole point of splitting them up in the first one was because 9 was an exact square. If there isn't an exact square then there is no way to simplify it further.(4 votes)

## Video transcript

Let's see if we can simplify 5
times the square root of 117. So 117 doesn't jump out at me as
some type of a perfect square. So let's actually take
its prime factorization and see if any of those prime
factors show up more than once. So clearly it's an odd number. It's clearly not divisible by 2. To test whether
it's divisible by 3, we can add up all of the digits. And we explain why this works in
another place on Khan Academy. But if you add up all
the digits, you get a 9. And 9 is divisible by 3, so 117
is going to be divisible by 3. Now, let's do a
little aside here to figure out what 117
divided by 3 actually is. So 3 doesn't go into 1. It does go into 11, three times. 3 times 3 is 9. Subtract, you got
a remainder of 2. Bring down a 7. 3 goes into 27 nine times. 9 times 3 is 27. Subtract, and you're done. It goes in perfectly. So we can factor
117 as 3 times 39. Now 39, we can factor as--
that jumps out more at us that that's divisible by 3. That's equivalent to 3 times 13. And then all of these
are now prime numbers. So we could say that this
thing is the same as 5 times the square root of
3 times 3 times 13. And this is going to be the
same thing as-- and we know this from our exponent
properties-- 5 times the square root of 3 times 3
times the square root of 13. Now, what's the square
root of 3 times 3? Well, that's the
square root of 9. That's the square
root of 3 squared. Any of those-- well, that's
just going to give you 3. So this is just going
to simplify to 3. So this whole thing is 5 times
3 times the square root of 13. So this part right over
here would give us 15 times the square root of 13. Let's do one more example here. So let's try to simplify 3
times the square root of 26. I'm actually going
to put 26 in yellow, like I did in the
previous problem. Well, 26 is clearly
an even number, so it's going to
be divisible by 2. We can rewrite it as 2 times 13. And then we're done. 13 is a prime number. We can't factor this any more. And so 26 doesn't have
any perfect squares in it. It's not like we
can factor it out as a factor of
some other numbers and some perfect squares
like we did here. 117 is 13 times 9. It's the product of a
perfect square and 13. 26 isn't, so we've simplified
this about as much as we can. We would just leave this as 3
times the square root of 26.